The present invention relates to a control member for a valve and method for determining fluid flow rate through a valve.
The invention has been developed primarily for a flow diverting valve used in controlling the pressure and flow rate, and measuring the flow rate, of a breathable gas supplied to the airways of a patient by a breathable gas supply apparatus during, for example, nasal Continuous Positive Airway Pressure (CPAP) treatment of Obstructive Sleep Apnea (OSA) and ventilatory assistance treatments such as non-invasive positive pressure ventilation (NIPPV). However, it will be appreciated that the invention is not limited to these particular uses and is equally applicable to controlling and measuring the flow of any fluid (ie. gas or liquid) passing a control member of a valve.
CPAP treatment is a common ameliorative treatment for breathing disorders including OSA. CPAP treatment, as described in U.S. Pat. No. 4,944,310, provides pressurised air or other breathable gas to the entrance of a patient""s airways at a pressure elevated above atmospheric pressure, typically in the range 3-20 cm H2O.
It is also known for the level of treatment pressure to vary during a period of treatment accordance with patient need, that form of CPAP being known as automatically adjusting nasal CPAP treatment, as described in U.S. Pat. No. 5,245,995.
NIPPV is another form of treatment for breathing disorders which can involve a relatively higher pressure of gas being provided in the patient mask during the inspiratory phase of respiration and a relatively lower pressure or atmospheric pressure being provided in the patient mask during the expiratory phase of respiration.
In other NIPPV modes the pressure can be made to vary in a complex manner throughout the respiratory cycle. For example, the pressure at the mask during inspiration or expiration can be varied through the period of treatment, as disclosed in the applicant""s international PCT patent application No. PCT/AU97/00631.
Typically, the ventilatory assistance for CPAP or NIPPV treatment is delivered to the patient by way of a nasal mask. Alternatively, a mouth mask or full face mask or nasal prongs can be used. In this specification any reference to a mask is to be understood as incorporating a reference to a nasal mask, mouth mask, full face mask or nasal prongs, unless otherwise specifically indicated.
In this specification any reference to CPAP treatment is to be understood as embracing all of the above described forms of ventilatory treatment or assistance.
Breathable gas supply apparatus broadly comprise a flow generator constituted by a continuous source of air or other breathable gas generally in the form of a blower driven by an electric motor. The electric motor driving the blower is typically controlled by a servo-controller under the control of a microcontroller unit. A hospital piped supply can also be used. The gas supply is connected to a conduit or tube, which is in turn connected to a patient mask which incorporates, or has in close proximity, a vent to atmosphere for exhausting exhaled gases.
International PCT patent application No. PCT/AU96/00586 (International Publication No. WO 97/10868) discloses a flow diverting valve with a rotatable control member, see FIGS. 19a and 19b of that application. The valve also has an inlet in fluid communication with a flow generator, an outlet in fluid communication with the patient""s airways and a vent in fluid communication with atmosphere. The rotatable control member disclosed can be positioned to close the vent and open a flow path from the valve inlet to the valve outlet for supplying gas during patient inhalation, and throttle or restrict the inlet and open a flow path from the patient to the vent to change the pressure or flow delivered by the circuit to the patient. The valve is thus suitable for use in a CPAP or NIPPV gas supply circuit.
It is an object of the present invention to provide an improved flow control member for a valve and, in a preferred form, a flow control member that allows simplification of an associated closed loop feedback control system and flow rate determination method.
Accordingly, in a first aspect, the present invention provides a control member for a valve, the valve having an inlet, an outlet and a primary flow path therebetween, the control member being rotatable to alter a control flow area of the primary flow path, wherein the amount of rotation of the control member between two predetermined end positions causes a substantially linearly proportional alteration in the control flow area of the primary flow path.
Preferably, at all rotational positions of the control member, the control area is the minimum cross-sectional area between the valve interior and the control member exterior downstream of the inlet.
The control flow area can be of any shape. In a preferred embodiment, the control flow area is a rectangular shape having a side defined by a wall of the valve interior, a side defined by a wall of the control member exterior, a bottom defined by the floor of the valve interior and a top defined by the roof of the valve interior.
The rotation of the control member is preferably controlled by a stepper motor, servo motor or other rotary actuator.
The two predetermined end positions preferably correspond to the control flow area being at a maximum and a minimum respectively. The minimum control flow area is preferably the primary flow path being substantially occluded or closed.
In a preferred form, the valve also includes a vent to atmosphere and a secondary flow path between the valve outlet and the vent, wherein the rotation of the control member is also adapted to alter a control flow area of the secondary flow path.
The control flow area of the secondary flow path is preferably the minimum cross-sectional area between the housing interior and the control member exterior upstream of the vent.
The control member is preferably adapted such that increasing the control area of the primary flow path simultaneously decreases the control area of the secondary flow path and vice versa.
Preferably, when the control flow area of the primary flow path is maximised or fully opened the control flow area of the secondary flow path is minimised or fully closed, and when the control flow area of the primary flow path is minimised or fully closed the control flow area of the secondary path is at least partially opened and more preferably fully opened.
In a second aspect, the present invention provides a method of determining fluid flow rate through a valve, the valve having an inlet, an outlet. a primary flow path between the inlet and outlet and a control member adapted to be moved to alter a control flow area of the primary flow path, wherein the amount of movement of the control member between two predetermined end positions causes a substantially linearly proportional alteration of the control flow area of the primary flow path, said method including the steps of.
sensing the fluid pressure at the inlet and the outlet;
sensing the position of the control member; and
determining the fluid flow rate from a predetermined function of fluid flow rate relative to the sensed fluid pressures and the sensed control member position.
Preferably, the control member is moved by being rotated.
The control member is preferably rotated by a stepper motor, servo motor or other rotary actuator.
In an embodiment, the valve also has a vent to atmosphere and a secondary flow path between the outlet and the vent. In this embodiment, the method includes the step of sensing the fluid pressure at the inlet, the outlet and the vent. In a variation of this embodiment, the vent is preferably substantially unrestricted and only the pressure at the inlet and the outlet is sensed because the pressure at the vent can be assumed to be at or close to atmospheric pressure.
In one embodiment, the predetermined function represents measurement of fluid flow rate at the outlet at predetermined control member positions over a range of is inlet, outlet and/or vent pressures. In an alternative embodiment, the function represents measurements of fluid flow rate at the outlet at predetermined inlet, outlet and/or vent pressures over a range of control member positions.
Once the outlet flow rate is known with respect to the inlet and outlet and/or vent pressures and control member position a preferred form of the method allows for the outlet flow to the patient to be determined by reference to the predetermined function. This advantageously allows determination of patient instantaneous flow without the need to provide for a separate flow meter to measure same. In a CPAP or NIPPV application, determination of patient instantaneous flow is often required to allow for the monitoring and use of related patient flow parameters such as tidal volume, minute ventilation, the phase of the respiratory cycle and patient airway flow limitation. This information may be established by reference to the patient instantaneous flow determined by reference to the pressure readings and control member position taken from the valve and the valve""s predetermined function.
The substantially linear relationship between the flow rate and pressure drop through the control area of the valve relative to the control member position advantageously facilitates the rapid and accurate feedback control of the valve and thereby allows for the valve to provide suitable pressure and flow transitions over a very short time interval, as is beneficial in NIPPV.